Modulation conversion system



Patented Apr. 25, 1950 UNITED STATES PATENT- OFFICE MODULATION CONVERSION SYSTEM Application December 5, 1947, Serial No. 789,788

4 Claims.

The present invention relates tosystems for deriving signals representing scanned subject matter such as an object or a representation in the form of a picture, document, printed matter, map, or other form, and, more particularly. but not necessarily exclusively, to novel apparatus for deriving signals suitable for operating a facsimile recorder to reproduce subject matter in facsimile.

The invention has for its principal aim or object to provide a novel system employing trigger circuits in a frequency demodulator thereby to provide a demodulator having a linear characteristic of wide range.

Another aim or object of thelnvention is to provide an improved method of converting a frequency modulated facsimile signal to an amplitude varying signal.

Another object of the invention is to provide a novel frequency meter especially useful in connection with a facsimile demodulator.

A still further object is to provide novel means for varying the output contrast between limits of a received signal to suit the characteristics of a facsimile recorder.

A still further object is to provide a novel method oi measuring frequency.

Other objects and advantages of the present invention will become apparent and immediately suggest themselves to those skilled in the art to which the invention is directed from a reading of the following specification in connection with the accompanying drawings in which:

Figure 1 is the schematic diagram of a frequency modulation to amplitude modulation section of a circuit embodying features of the in vention.

Figure 2 is a schematic diagram of a frequency meter circuit embodying features of the invention.

The received facsimile signal may be detected and amplified in a radio receiver (not shown) and fed as a frequency modulated signal to the primary It of a transformer it. This transformer serves as a step-up transformer and impedance matching device to match the line impedance to the impedance of a filter M. The filter I4 is a band-pass filter passing only the desired band and eliminating any extraneous frequencies. The desired signal band is then fed through a transformer It to the grid 11 of a tube l8. The tube l8 and tubes I9, 20 and M and their associated components constitute an amplifier and a cascade voltage amplitude limiter having special provisions to insure that the limited output signal is a square wave over a wide range of input levels. The duration of each pulse of this square wave signal is equal to the duration of the intervals between pulses so that a 50/50 square wave results.

Referring to Fig. l of the drawings there is shown diagrammatically a demodulating system in accordance with the invention. An accompanying modulatin system is also shown. Ar-

10 rangements for generating a frequency varying signal are disclosed in U. S. Patent No. 2,257,282, granted to J. E. Smith et al. on September 30, 1941. Another arrangement for modulating a tone with image signals to provide a frequency 16 modulated tone or carrier is shown in U. S. Patent No. 2,321,269, granted to Maurice Artzt on June 8, 1943. The frequency modulated signal source may be a transmitter modulated by speech and the signals may occur in bursts.

2c The frequency modulated input, for example a facsimile signal, is applied to the first stage of the limiter by way of a coupling transformer 25, the secondary 26 of which is connected to the grid 28 of the first limiter tube is by way of a switch 36. The purpose of this switch will be explained hereinafter. The grid circuits of the limiter tubes 19, 20, and 2!! are provided with resistors 32, 33 and 3d. The incoming signal, for example the signal wave appearing at the input I0, alternatively drives the limiter tubes from beyond cut-0d to grid current limiting to produce the square wave. The anodes of the tubes'ld to 28 are connected to the positive terminal of a suitable direct current source 35 not shown) by way of the terminals 36. The cathode 38 of the tube is is connected to the negative terminal of another direct current source (not shown) the cathode of the tubes 26 and 29 are grounded.

it The square wave output from the limiter is applied to the grid 39 of a phase inverter tube M. The outputs from the tube at are taken across a plate resistor Kit and a cathode resistor d6 so that these two outputs are 180 out of phase; Resistors tit and d5 are grid return resistors. It will be understood that other phase inversion arrangements may be employed if desired instead of the particular phase inverter arrangement shown.

In efiect, when the output across the resistor 43 is in the negative direction, then the output across the resistor 85 is in the positive direction. The negativeportions of these two square waves are difierentiated by differentiator cir-.

cuits comprising the capacitors 38 and t9 and i 3. I the resistors KI and 62 the common terminal-of which is grounded. It will be understood that the output appearing across the resistor 43 is difierentiated by the difierentiating circuit 485I and the output appearing across the resistor 46 is difierentiated by the difierentiating circuit 4952. The negative pulses resulting from this simultaneous differentiation are employed for controlling the trigger circuit 54. At every transition of the signal a pulse is formed,

66 is conducting. The about 6.7 times the reciprocal of twice the highso that there are two pulses for every cycle of 1 signal frequency.

The manner of obtaining and applying the pulses will now be'described. The differentiated output from the capacitance resistance combination 485I is applied to the cathode 56 of .a diode 58. The difierentiated output of the capacitance resistance combination 49-52 is ap-.

plied to the cathode 59 of a diode 63. The plates of the diodes 58 and 63 are connected to one terminal of a resistor 66.

The trigger circuit 54 heretofore referred to is shown, by way of example, as comprising a trigger circuit having a pair of tubes 68 and 68. These tubes are connected in the usual manner to provide a tripped condition of definite time duration. The tube 69 is normally conducting causing a large negative voltage on the grid 'II of the tube 68 keeping it non-conducting. For example, when the trigger circuit is tripped, the tube 68 is rendered conducting with tube 69 nonconducting. The time cycle of the self-restoring trigger circuit is such that the tube 68 again conducts after a definite time period, when tube 68 is rendered non-conductive until tripped by a pulse. The time duration of the tripped condi. tion of the trigger circuit is determined for the most part by a coupling capacitor I2 and a resistor 14. A resistor I6 is connected to ground. A diode I8 is included in the cathode circuit of the tube 68 so that cathode current fiows through this diode when the tube 68 conducts.

The'diodes 58 and 63 conduct successively when negative pulses are applied to their cathodes, a common conductive path for the diodes being provided by the previously mentioned resistor 66. As either diode conducts a negative pulse is applied to the cathode 19 of the tube 68. After a nal represents substantially the h ghest deviation frequency. For this condition the pulses follow each other rapidly and their average output.

A demodulator of the character just described is shown and claimed in a copending application of Charles N. Gillespieentitled "Conversion of frequency modulated signals, Serial No. 685,589, filed July 23, 1946.

The operation of the trigger circuit 64 as a demodulator will be reviewed more in detail at this point. Whenever a negative pulse is coupled through the diode 58 to the cathode 119 of the tube 68, this tube starts to conduct through the diode i8 and a negative pulse is coupled from the plate of the tube 68 to the grid 8i of the tube 69 and causes it to become non-conducting. The time constant circuit comprising the resistor 16 and the condenser '12 controls the length of time for which the tube 69 is non-conducting and the tube time may be, for example,

est deviation frequency. The result is expressed in seconds as the unit. A resistor 82 is used to return the grid 'II of the tube 68 to a potential slightly more positive than zerowhen this tube (68) starts to conduct. Each time the tube 68 is shut off, a positive pulse is coupled through a condenser 83 to the grid 84 of a tube 86. Each time the tube 68 conducts, a negative pulse is fed to the tube 86. The time duration of the negative pulses to the tube 86 is substantially constant for all applied deviation frequencies. The number of pulses increases with frequency. Every negative pulse causes the tube 86 to become non-conducting and remain of! for a fixed length of time.

A positive signal of fixed duration is thus fed into a filter 88. Filter 88 is a low-pass filter, attenuating all but signal frequencies. At higher frequencies, more of these positive signals are fed into this filter. A demodulated D.-C. output suitable for operating a facsimile recorder of the type operable in response to a demodulated signal is available at the terminal 86.

The output signal from the filter 88 is fed into a voltage divider and bias corrector arrangement comprising a series arrangement of resistors 9i to 96 and a second series arrangement 81 to I63. Contacts I66 are connected between the resistors 9| to 96 and contacts I61 are connected to the resistors 91 to I63. A movable switch bar I69 connects a selected pair of contacts I66 and I61. The output switch I69 may be moved to select certain percentages of the output signal from the filter 88. As the switch selects larger percentages of the filter output voltage, the negative voltage bias is increased. The compensating set of resistors 91 to I63 are switched in so that the bias on the grid II I'of a tube II2 remains constant for the FM carrier frequency (center of the deviation band) no matter which position the switch I66 is in. This switch controls the output AM contrast. Diflerent facsimile recorders (not shown) which may be used and which are to be connected to the system output I56 require diiferent contrasts between black and white to operate properly.

The tube H2 and a tube II4 constitute an AM modulator. The modulating signal is brought in on the grid III of the tube H2. The carrier signal is generated by an oscillator comprising tubes H6 and Ill. The grid I2I of the tube II! is connected to the negative terminal I36 of a D.-C. voltage supply source (not shown) and its plate is connected to a tank circuit I 22 and I24. The tank circuit is grounded but the reference potential at this point is more positive than the grid I2I. The cathodes I26 and I28 of the tubes II! and H6, respectively, are connected to the terminal I36 through a resistor I3I. One stage drives the other through the cathode resistor I3I. The plate of the tube II 6 is connected to ground through a resistor I34 and its grid is connected to the terminal I36 through two resistors I36'and I34. The output of the tube II! is coupled back to the input of the tube II6 by way of a condenser I33. An oscillator similar to the one just described appears in Electronics, for May of 1946. The signal generated by the oscillator is coupled to the grid I of the tube H4.

The tubes H2 and H4 are coupled together through a common cathode resistor I43. The amplitude of the carrier signal from the oscillator is sufficiently largeto cut oil the tube H4.

varies. -An arrangement of this character is disclosed and claimed in a copending application for Letters Patent of the United States, filed July 9, 1947, by E. R. Shenk, entitled Amplitude modulator," and identified by Serial No. 759,891, now U. 8. Patent No. 2,469,747, dated May 10, 1949,

The AM signal fzom the tube H4 is fed into a band-pass filter I49 which passes only the carrier and its side band. -R eSistrs I5l and I52 serve as terminating resistors for the filter I48 and, also. as a voltage divider. The filtered signal is amplified in a tube I54 which has the primary I55 of a transformer I56 included in its plate circuit.

A meter I51 for indicating decibels is connected across the transformer secondary I50 to read' the db contrast from blaok to white. The meter I51 together with the contrast switch I09 enable the AM signal to be adjusted to operate properly with a selected facsimile recorder (not shown). Terminal equipment other than the transformer I58 may be employed, if desired.

Figure 2 of the drawings discloses a frequency meter circuit which may b employed to measure the frequency limits of the signal, which in the illustrative embodiment is obtained from the plate of the tube 2! A'terminal I8! is or may be provided for readily obtaining access to the plate connection of this tube. When the apparatus of Figure 1 is to be used with the apparatus of Figure 2, the terminal IBI may be connected to one contact I82 of a switch I84. The remaining contact I80 of the switch is available for other purposes such as the application of a transmitted signal to the frequency meter circuit of Figure 2 as shown in the copending application of the same inventors entitled Pulse type modulator for facsimile signals, filed November 29, 1947, Serial No. 788,790.

A coupling condenser I 88 and a resistor I89 differentiate the square wave input. A differentiated output of this R.-C. circuit is fed as pulses to the cathode I9I of the tube I92. A tube I99 has its plate I96 and cathode I91 connected-inseries with elements of the tube I92 to ground.

The tube I 92 together with the tube I99 and their associated components constitute a trigger circuit with the tube I99 normally conducting and the tube I92 normally non-conducting. The grid 20I of the tube I92 is normally held very negative with respect toits cathode. Bias for this apparatus is supplied from a terminal 203 which is or may be connected to the same point of a suitable voltage supply as the terminal 30 of Figure 1. When a negative pulse is applied to the cathode I! of the tube I92 it causes thi tube to conduct. The tube I94 supplies the electrons through a low impedance path to keep th tube I92 conducting as long as desired. I

The grid 206 of a tube 201 is connected to a resistor 208 to the grid 20I of the tube I92. The tube 201 conducts and is non-conducting at the same time as the tube I92. Every time the tube 201 conducts, a fixed amount of current is passed through this tube and through a milliameter 2I I.

As the frequency of the measured signal increases, the number of times current is passed through the milliameter is increased. The current through the milliameter thus increases linearly with frequency.

The plate of the tube 201 isconnected to suit;- able sources of plate voltage (not shown) through a resistor 2I2 and an adjustable resistor 2 I 4 As- I ,sumingthat a milliameter 2 II is used in the cathode circuit of the tube 201 then with a proper value of resistance in the plate circuit of the tube 201 the circuit can be adjusted so that at 2'kilocycles two milliamperes will flow through the meter 2. The meter will then read kilocycles directly as well as milliamperes.

The switch 3|, previously mentioned will switch into the receiving circuit a test tone of a predetermined frequency, when the switch is moved from its contact 22I to the contact 223. The tone is used to calibrate the frequency meter. The adjustable resistor 2 I4 is adjusted until the meter reading is thesame as the test tone frequency. The switch 3i is mechanically connected to a switch 226 as indicated at 228. The switch 220 shunts a condenser 23I across the condenser I22 and changes the oscillator frequency from the carrier frequency to the test ton frequency. The operation of the apparatus of Figure 1 will, it is believed, be understood from the detailed description of the foregoing. However, the operation will be reviewed briefly at this point. Frequency signals from any type of facsimile scanning systern or from receiving terminal equipment are applieTto the primary I0 of the transformer I2. This input signal is limited by the tubes i8, I9, 20 and 2!. The tube 4i serves as a phase inverter to apply the amplified signals to a demodulator comprising the diodes 58, 63 and 18 and the tubes 68 and 69. This demodulated signal is amplified by a tube and filtered by a, filter 88.

The filtered signal is applied to an amplitude modulator comprising tubes H2 and lid through a contrast control device comp; ising a swiich bar I09. The modulator is provided with a carri r signal from an oscillator comprising the tubes H6 and Ill. The amplitude modulated output from this modulator is available for operating facsimile recording devices.

The oscillator also is available for applying a calibrating frequency to the limiter tubes for calibrating the frequency meter shown by Figure 2 of the drawings.

Having now described the invention, what is claimed and desired to be secured by Letters Patent is the following:

1. A facsimile system for converting a frequency modulated signal to a signal suitable for operation of a facsimile recorder, comprising an input filter, a limiter connected to said filter, an FM demodulator connected to said limiter, a filter connected to said demodulator, a contrast control apparatus connected to said filter, a modulator connected to said contrast control apparatus, an oscillator coupled to said modulator, said modulator having a filter in the output circuit thereof, and means for coupling said filter to said circuit, said output circuit including means for observing operation of said contrast control.

2. A facsimile system for converting a frequency modulated signal to a signal suitable for operation of a facsimile recorder, comprising an input filter, a limiter connected to said filter, an FM demodulator connected to said limiter, a filter connected to said demodulator, a contrast control apparatus connected to said filter, a modulator connected to said contrast control apparatus, an oscillator coupled to said modulator, said modulator having a filter in the output circuit thereof, and means for coupling said filter to said circuit, said output control. l

4. A facsimile system for converting a frequency 1 circuit including a meter for observing operation of said contrast control.

3. A facsimile system for converting a frequency modulated signal to a signal suitable or operation of a facsimile recorder, comprising a input filter, a D.-C. coupled limiter connected tosald filter, an FM demodulator connected to said limiter. a filter connected to said demodulator, a, contrast control apparatus connected to said filter, a modulator connected to said contrast control apparatus, an oscillator coupled to said modulator, said modulator having a filter in the output circuit thereof, and means for coupling said filter to said circuit, said output circuit includin a meter for observing operation of said contrast modulated signal to a signal suitable for operation of a facsimile recorder, comprising an input filter, a D.-C. coupled cascade limiter connected to said filter, an FM demodulator connected to said limiter, a filter connected to said demodulator, a contrast control apparatus connected to said filter, a

modulator connected to said contrast control apparatus, an oscillator coupled to said modulator, said modulator having 'a filter in the output circuit thereof. and means for coupling said filter to an output circuit, said output circuit includin a meter for indicating decibels to provide data said contrast control. EUGENE Rr SHENK. ANTHONY LIGUORI.

REFERENCES crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS directly for operation of 

